(Original post by SunilP_123)
It's got nothing to do with lower bounds. Its just the upper bound of the side opposite to the angle. Other sides don't natter.

I did this and got 97.7...From the cos graph, between 0 and 180 degrees, the lower the cosine value, the higher the angle. Therefore, to get the maximum angle, you want to make your cosine value as low as possibleHere you could, so because (between 90 and 180) the angle would be larger if the cosine value was more negativecos B = (BC^2 + AB^2 - AC^2)/ (2BC x AB)Since the - AC part is negative, you want to use the maximum value of AC (20.5).The earlier part of the question was a hint to use 11.5.Then if you try 15.5 and then 16.5 for the remaining side, it turns out that 15.5 gives the larger angle, which was 97.7 (3s.f.)